1. Field of the Invention
This invention relates to a pressure-sensitive sensor capable of detecting an action of external force at a high sensitivity through an elastic deformation of an optical wave guide and having excellent heat resistance and impact resistance. Furthermore, it relates to a method of producing an optical wave guide used for such a pressure-sensitive sensor, and more particularly to a method of producing an optical wave guide having sufficiently improved optical transmission properties with preventing ray scattering loss and others at a boundary portion between core and clad.
2. Related Art Statement
In order to detect the presence or absence of external force or a magnitude thereof, there has hitherto been known a pressure-sensitive sensor as disclosed in Japanese Patent laid open No. 55-133,709, wherein a pair of electrodes are closely arranged on opposite surfaces of pressure conductive rubber to form a contact mechanism and a lead wire is connected to each of the electrodes and the contact mechanism is airtightly covered with an insulative coating to thereby form a tape-like pressure-sensitive sensor.
In general, the tape-like pressure-sensitive sensor serves to operate on-off control of motor or the like connected to a switching circuit when an electric resistance value of the pressure conductive rubber gradually decreases and reaches below a given value as the external force acting to the pressure conductive rubber increases.
In this pressure-sensitive sensor, however, there are problems that when the pressure conductive rubber is subjected to a repetitive deformation such as compression, bending, torsion or the like, metallic powder and other conductive particles dispersedly incorporated in the inside of the pressure conductive rubber fall down from the rubber at a relatively early time and also the electrode made of metal plate, metal thin sheet, conductive film, deposition film or the like suffers premature breakage or other injury through the repetitive deformation.
Further, when the breaking of the lead wire or the other trouble is caused in the pressure-sensitive sensor viewing from its structure, or when a power supply is accidentally stopped, even if the external force is actually applied to the pressure-sensitive sensor, the signal of detecting the external force can not be output, so that when such a pressure-sensitive sensor is used in a robot or other safety device, there is a problem of causing a serious accident. That is, when the pressure-sensitive sensor is used as a safety device, if abnormal situation is caused, the safety device is required to actuate toward a side of guaranteeing safety or warn self abnormal state. In this case, however, the conventional pressure-sensitive sensor is at a similar state as in the case of inaction of external force even if the external force acts to the sensor, so that the function as a safety device can not sufficiently be developed.
In order to solve these problems, Japanese Patent laid open No. 61-34,493 proposed a pressure-sensitive sensor comprising a flexible optical fiber, a light emitting means for introducing a light into the optical fiber and a light receiving means for receiving a light transmitted inside the optical fiber and converting a voltage signal proportioned to a light receiving quantity into a binary code as compare with a threshold value, which detects such a feature that the optical fiber is curved at a contact state with an object by the increase of optical loss.
Since such a sensor is an optical sensor, it is not necessary to perform the dispersion mixing of conductive particles or adhesion of electrode, so that there is no fear of causing the former problems described in the pressure-sensitive sensor using the pressure conductive rubber. Furthermore, in case of occurrence of abnormal situation such as trouble of sensor or accidental power supplying stop, the quantity of light arrived at the receiving means decreases or disappears likewise the case of acting the external force to the sensor, so that when this sensor is used as a safety device, the safety can sufficiently be ensured in the occurrence of abnormal situation.
However, the pressure-sensitive sensor as an optical sensor uses a quartz fiber, multi-component glass series fiber or plastic fiber as an optical fiber. Although this optical fiber has a flexibility, the rigidity becomes too high and hence the freedom degree of deformation in the fiber is low, so that it is impossible to produce deformations such as a large compression deformation in a direction perpendicular to the axial line, a partially large bending deformation and the like.
Therefore, there are the following problems:
(1) Even when the external force is applied to the pressure-sensitive sensor, the deformation amount of the sensor and hence the radiation loss is small, and consequently the sensitivity of the sensor is very low;
(2) When the sensor is excessively deformed, the breaking, plastic deformation and the like of the optical fiber occur; and
(3) Since the optical fiber is not substantially subjected to elongation deformation, in case of bending deformation of the pressure-sensitive sensor, the peeling of the optical fiber from the light emitting or receiving means is caused due to a large component of force in a tensile direction.
Particularly, when using the optical fiber made of a plastic fiber, the softening point of the plastic fiber is within a range of 80.degree..about.125.degree. C., so that it is impossible to apply the pressure-sensitive sensor to a place exposed to a high temperature above 80.degree. C. On the other hand, the glass transition temperature of the plastic fiber is usually about room temperature and hence the properties of the plastic fiber rapidly degrade at a temperature below room temperature, so that there is a problem that the lower limit of use temperature of the pressure-sensitive sensor is considerably restricted.